Current opal assessment and evaluation practices are highly subjective as they are based on a combination of human observed factors, such as scoring the flashes of colour as the opal is moved, the body tone, colour, brightness and pattern. The fact that Australia contributes 95% of the world's opals is both a blessing and a curse. Overseas buyers often do not have the training to assess the value of opals being supplied (leading to difficult negotiations between buyer and seller), nor can they describe a particular type of opal in sufficiently concise and objective terms for the suppliers to be able to provide the appropriate opals.
Most gemstone assessment is made using a magnification 10× loupe. The gemstone is observed by holding it in a pair of gemstone tweezers and turning it (using pitch, roll and yaw) to observe light interaction with the gemstone and external and internal characteristics.
The subjective nature of current opal assessment and evaluation practices creates difficulty in negotiations and substantial distortions in the terms of trade between the opal miners and the buyers.
There exist a number of key characteristics on which an opal can be graded for its value, such as:                Colour (hue) and area of flash        Brightness        Body tone        Pattern        Shape        Other characteristics        
In terms of relative importance to the overall gemmological value of an opal, colour and body tone combined have about a 40% weighting, followed by brightness and pattern, each at about 30%, with the remaining characteristics being of much less significant weighting. In terms of seeking an objective assessment by a human observer, colour appears to be the most difficult, followed by brightness, which seems easier to assess and quantify manually, as is the pattern, of which there are 28 main types.
Colour is difficult for a human observer to assess because of the following:                (1) The contribution of each colour changes with viewing angle, that is, with pitch, roll and yaw. All orientations are to be integrated in a colour estimation, which is difficult due to the limited perception and subjective memory of humans and their eyesight variables.        (2) Out in the field, it is difficult to accurately assign a shade according to a colour reference chart because the conditions for viewing are highly variable.        
To compound the problem, synthetic or man-made replicas of opals and other gemstones are improving, and in some cases (eg diamonds), it is extremely difficult to detect the natural gemstone from the synthetic. Verifying the authenticity of the gemstones is another problem that may be resolved by adoption of objective and automated analytical methods.
One approach to provide an objective and automated analytical method and image capture device for the grading of diamonds is disclosed in U.S. Pat. No. 6,239,867 (“the Patent”). Although also referring to use of the method and device for the grading of opals and other gemstones, that approach is unsuited to opals and is poorly suited to other gemstones, even diamonds. The image capture device and method disclosed in the Patent do not allow for gemstone movements of pitch, roll and yaw during the capture of images so as to display the “play of colours” of the gemstone, as the gemstone stage is not tiltable. Nor does the Patent disclose the assessment of all segments of the face of a gemstone for each of colour, brightness, body tone and other characteristics, many of which are especially important for opals, before grading the gemstone for each characteristic. Rather, the Patent discloses a method of sampling a small area of a diamond by moving a camera through a controlled arc and averaging the total illumination data in that area to assess the colour of the diamond.
Such a limited sampling is not suited to opals as it will not display the “play of colours”, body tone and brightness of an opal over all segments. In particular, the Patent states that gemstone colour analysis is done by obtaining average red, green and blue (RGB) values for colour images in the image pixel region delineated by the girdle and the table facets, and that by sampling the colour of a smaller region a more predictable and accurate colour reading is obtained. These procedures are not suited to the assessment of opals, where it is the entire face of the opal that requires colour assessment.
Furthermore, the image capture device disclosed in the Patent filters the light between the gemstone and its camera, and this is stated to be critical to the analysis of colours. That device also uses high viscosity immersion oil either between the gemstone and a glass plate upon which it is centrally placed or to immerse the gemstone so as to remove glint and assist in the transmission of light though the gemstone for the detection of flaws or colour inclusions. These features of the image capture device and method disclosed in the Patent are unsuited to opals and are poorly suited to other gemstones.
The beauty of coloured gemstones depends entirely on their effect on light. Factors attributing to the assessment of a gemstone will include colour, changing colour patterns, transparency, lustre and brilliance, dispersion and “fire”.
Coloured gemstones are separated into crystal systems defined by their atomic structure, geometric form as an expression of the atomic structure, and the relationship between the structure and physical properties.
Each crystal system has defined optical properties. They can be:                singly refractive—(isotrophic) where a refracted ray travels in a single new direction at a constant new velocity        doubly refractive—(anisotrophic) where a light ray is refracted and split into two rays which take different paths and proceed with different velocities, the medium having more than one refractive index        doubly refractive uniaxial—where such a medium possesses one direction parallel to which the two split rays appear to be isotrophic—the optic axis        doubly refractive biaxial—where such a medium possesses two directions parallel to which the two split rays appear to be isotrophic.        
The following Table identifies the various geometric forms that different crystal systems take:
Crystal SystemExamplesOptical CharacterCubicdiamond, garnet, spinelIsotrophicTetragonalzircon, scapoliteAnisotrophic UniaxialHexagonalemerald, apatiteTrigonalSapphire, ruby, tourmalineOrthorhombicTopaz, peridotAnisotrophic BiaxialMonoclinicKunzite, moonstoneTriclinicSunstone
Coloured gemstones are further defined by their chemical composition and trace elements which affect colours produced and the behaviour of light, both reflected and refracted, and are identified by their optical properties, specific gravity, hardness, inclusions and colour using various known instruments. Such instruments include:                10× hand lens—used for routine observation of gemstones        Refractometer—measures the refractive index of light emitted through the stone        Dichroscope—shows two colours present in a doubly refractive stone        Polariscope—determines the optical character of the stone—doubly or singly refractive, and assists to find the various crystal axis of the stone        Microscope—analysis of internal characteristics by extended magnification        Specific gravity—displacement measurement in water        Chelsea filter—allows transmission of both deep red wavelengths around 690 nm and yellow-green wavelengths, around 570 nm, that matches emerald's emission and absorption characteristics, recommended to assist the discrimination between natural emerald and its simulants such as green glass, tourmaline and peridot.        Spectroscope—measures the spectral emission and interference wavelengths        SW/LW UV light box—measures the stone's reaction to UV light—fluorescence and phosphorescence.        
Coloured gemstones are assessed on the quality of light refracted and reflected—brightness and saturation of colour, inclusions present, quality of cut and carat weight; the four “C”s as noted by the diamond industry—colour, cut, clarity and carat weight.
Coloured gemstones have faceted surfaces in various shapes—round, rectangle, square, pear, heart, triangular being more common (virtually any shape is now possible particularly with laser cutting), cut and polished en cabochon—oval or round, beads or tumbled, carved and freeform shapes.
Almost any colour can be found among gemstones, and the uncut minerals from which they are derived. Some species show a wide range of colour whereas others are constant.
Gemstones are coloured by the selective absorption of light—the interaction between light waves and electrons and/or structural colouring. For idiochromatic (self coloured) minerals, their colour is a fundamental property, constant and characteristic of that mineral and the colour is due to the major chemical composition of the mineral. When the colour arises from a trace impurity in the basic chemical composition, the mineral is called allochromatic (other coloured).
Pleochroism or dichroism is the change in colour evident as the mineral is rotated under plane-polarized light. The primary cause of dichroism or pleochroism in minerals is due to adsorption of particular wavelengths of light. This selective adsorption of certain wavelengths of light causes the transmitted light to appear coloured. This colour is a function of the thickness and the particular chemical and crystallographic nature of the mineral. If the adsorption of particular wavelengths of light differs according to the optical path, the phenomena of pleochroism is evident. This is observable in plane-polarized light when the polarizers select the light exiting from the mineral. This colour depends on which optical path is viewed. Pleochroic colours are observed and recorded in the mineral data and are generally diagnostic of the particular mineral.
Coloured gemstones exhibit various degrees of transparency, these being:                transparent—an object viewed through the stone shows outlines clearly and distinctly        a translucent—some light passes through but no object can be seen through the stone        opaque—no light passes through        
Lustre is a basic essential character of cut and polished gemstones. Lustre imparts life and brilliance and has much to do with their beauty. Lustre is purely an external surface effect and depends on the amount and quality of reflected light from the surface of the gemstone.
The various types of lustre are described in the following Table:
LustreDescriptionExampleAdamantineHard and brilliantDiamondVitreousLike broken glassEmerald, rubyResinousAs resinAmber, opal, somegarnetsWaxySeems covered with a thin layerTurquoiseof oil. Can result from thescattering of light by amicroscopically rough surfacePearlyOverlapping parallel lamellaeMoonstoneSilkyFinely fibrous structureGypsumMetallicThe brilliant appearance of metal.Gold, pyrites
Sheen is due to reflection of light from below the surface of the coloured gemstone. It is caused by the internal structure of the gemstone and there are several distinctive types, as described in the following Table:
SheenDescriptionExampleIridescenceRainbow effect often observedQuartz, calcite, irisin cracks and flaws in a stoneagate, labradorite,orient of pearlLabradorescenceDiffraction at alternatingSpectrolite,layerslabradoriteAdularescenceSheen of silvery light crossingmoonstonethe gemstone caused byalternating layers of differentfeldsparsOpalescenceScattering of light from smallCommon opal/particles in the stone, milkyopal glasstranslucent effectPlay of ColourUsed to describe the coloursOpalseen in precious opalChatoyancyReflection effect due toQuartz, tigers-eye,parallel fibres, tubes orchrysoberylneedles of included substanceson a cabochon cut stoneAsterismMobile star effect fromRuby, sapphire,parallel fibrous inclusionsalmandine garnet,aligned with specific crystalchrysoberyl, diopsidedirectionsAventurescenceSpecular reflections orAventurine quartz,spangles of light reflectedsunstonefrom plate like inclusions
Inclusions which interfere with the transfer of light through the coloured gemstone may reduce its value, whilst other inclusions prove its authenticity, yet other inclusions provide a welcome effect such as the layered rutile needles present in star sapphire and ruby which if present, cause a pleasing star effect to appear on a cabochon cut corundum (sapphire and ruby) or the chatoyancy of cats eye chrysoberyl.
Traditionally, colour grading at gemological laboratories is carried out by the human eye. For instance, when colour grading a diamond, a gemologist will line up a master-set of diamonds (featuring a D to Z colour) to which the diamond's colour is compared. The master-set of stones and the diamond are placed on a dull white countertop. The master-set is lined up with lightest (the top colour) first, becoming progressively darker. The diamond's colour is then compared to the colour of each stone of the master-set until the grader perceives its colour to be the same.
Several problems arise when a human observer grades a diamond: every eye has a different “colour deficiency”. This means that every grader has deficiencies for certain colours. Further, the colour and intensity of illumination influences the eye's perception. This results in deviations in colour grading of large diamonds (a few carats) and small diamonds (a few tens of points) as most master-sets feature diamonds of around one carat in size. Finally, larger labs will use two different master-sets with inevitably slightly different colours. As such, colour grading within the same lab or between its branches may vary.
There have been numerous attempts to provide standardised reporting systems for the colour grading of gemstones. One prominent system is provided by the Gemological Institute of America (GIA). Its report provides detailed information about the dimensions, carat weight, shape and cut of a gemstone. GIA reports also cover any treatments or enhancements that have been done and of course, whether the stone is natural or synthetic. A GIA report utilizes a standardised “coloured stone grading system” chart to graph the hue, tone, and saturation of a stone. Gemstone colour grading is broken into three quantifiable categories: intensity (saturation), hue (colour), and tone (lightness/darkness).
The GIA specifies and labels up to thirty one gemstone hues according to the gemstone hue chart. The “hue” is the actual “colour” of the material (blue, green, red, etc.).
The GIA nomenclature also specifies six levels of saturation ranging from “grayish” (neutral grey) to “moderately strong” to “vivid” according to a gemstone saturation chart. The term “intensity” is also used to describe a stone's saturation.
To describe the darkness or lightness of a coloured stone, the GIA system has nine levels of tone ranging from “very very light” to “very very dark” according to a gemstone tone chart. A numerical value is assigned to each label for use in a grading report.
With the exception of hue, each of these parameters by themselves are relatively meaningless and they must be used together to meaningfully reveal a gemstone's colour quality. Taken together, these three parameters will provide a simple numeric code that can quickly and accurately quantify gemstone colour.
Reports are also provided by the American Gem Trade Association and by American Gemological Laboratories. All of these reporting systems, however, use different criteria and standards of colour grading and so can be confusing when considered together.
Alongside colour quality as a criteria for grading gemstones, brilliancy is the amount of flash the gem returns to your eyes. Most diamonds are 100% brilliant. An average brilliancy of 50% means half of the stone returns flash.
The Gëbelin Gem Laboratory in Lucerne, Switzerland (GGL) is one of the leading testing laboratories in Europe, testing coloured stones, fancy coloured diamonds, colourless diamonds, and pearls. They also test diamonds for any sign of (HTHP) enhancement. Coloured stones are tested for the usual parameters of clarity, colour, cut, and enhancements as well as place of origin, Diamonds are tested for clarity, colour, fluorescence, cut, and symmetry.
Generally, the grading criteria for coloured gemstones, in order of decreasing importance, can be summarised as comprising the following:
Colour
                Hue        Saturation        BrightnessCut        Shape        Proportion        SymmetryClarity        Inclusions mapped        Verification of type as pertaining to gemstone variety        Degree of transparency of the gemstoneCarat Weight        As per industry standardScintillation        Surface reflection/refraction from smaller facets—“twinkling” effect as the stone is movedBrilliance        Return of light from the gemstone which has been internally reflected and hence has colour produced by filtering through the coloured material of the gemstoneLustre        Surface feature of the gemstone—analogous to the fineness of polish; high lustre produces sharp edged surface reflections while low lustre produces dull or fuzzy edged surface reflectionsDispersion        Resulting from the breaking up of white light into its spectral components—more characteristic of diamonds than other coloured gemstones, coloured gems show low dispersionSheen        Play of light due to the internal characteristics of the stone        